The present invention relates to sensing and signal processing devices based on acoustic waves propagating in solids. More particularly, it has been discovered that a new type of acoustic wave, referred to as a quasi-SH (shear horizontal) acoustic wave, can propagate in thin elastic plates if the plate thickness is comparable in value to the acoustic wavelength. This wave has many attractive properties. Devices employing this wave will therefore provide many advantages over those using other types of acoustic waves.
Devices based on acoustic waves propagating in solids find many applications in electronics. The main applications of these devices are in the areas of signal processing and sensing. Signal processing devices include delay lines, resonators, filters, oscillators, convolvers, etc. Acoustic wave delay lines and resonators can also be used for a variety of physical, chemical, and biological sensing applications. Acoustic waves propagating in solids can be broadly classified into three types. These are (i) bulk acoustic waves, which are elastic waves propagating inside solids, (ii) surface acoustic waves, which are acoustic waves propagating at the free surface of a semi-infinite elastic medium, and (iii) Lamb waves or plate waves which are acoustic waves propagating in a plate of finite thickness.
The present invention is concerned with acoustic waves propagating in plates of finite thickness. In particular, the present invention discloses a new type of acoustic wave which is nearly polarized in the shear horizontal direction, and which can propagate in thin elastic plates if the ratio h/.lambda. k is less than about 3, where h is the plate thickness and .lambda. is the acoustic wavelength.
Attractive properties of this quasi-SH wave include: (i) nearly nondispersive propagation, (ii) very large value of K.sup.2, the electromechanical coupling coefficient, and (iii) ability to propagate in contact with a liquid medium without suffering excessive attenuation. The nondispersive propagation and large value of K.sup.2 makes the wave attractive for use in signal processing and sensing devices. Further, the fact that we wave can propagate in contact with a liquid medium allows the wave to be used in sensors that have to operate in contact with a liquid medium.
Two previous patents have discussed the use of acoustic waves propagating in thin plates. The patent by Martin el al U.S. Pat. No. 5,117,146 is concerned with pure shear horizontal (SH) waves. The limitations of this patent are: (1) SH waves exist in a very limited number of materials, namely isotropic materials, and a very limited number of other materials with very special crystal symmetries, and (2) the coupling coefficient, K.sup.2, of these waves is very low. For example, the material used by Martin et al, namely ST-cut quartz can support pure SH waves. But the value of K.sup.2 in this material is only 0.0026. In contrast, the quasi-SH wave disclosed here exists in any arbitrary material (the material need not have any special crystalline symmetry) and it also can have very large value of K.sup.2. For example, values of K.sup.2 as high as 0.15 have been obtained in Z-cut, X-propagating lithium niobate, and still higher values are possible in other materials. Another point to be noted is that in the deuces proposed by Martin, one gets a large number of resonances due to the multiple modes that can propagate in the plate. This makes it difficult for the sensing oscillator to lock onto a particular mode.
Another patent on Lamb waves is that by White et al U.S. Pat. No. 5,129,262. This patent is concerned with the lowest order antisymmetric Lamb wave mode, commonly referred to as the A.sub.o mode. In order to use the A.sub.o mode for sensing in liquids, the velocity of this mode must be less than the velocity of sound in the liquid. This typically requires very small values for the ratio h/.lambda. (usually less than 0.05). There is no such restriction for the quasi-SH wave that is disclosed herein. For example, the value of h/.lambda. could be quite large, greater than 0.5 or so, and yet the device can operate satisfactorily in a liquid medium.